This invention relates generally to electroluminescent lamps and, more particularly, to a controller for driving panels comprising such lamps at optimal frequencies.
Electroluminescent Lamps:
An electroluminescent (EL) lamp generally includes a layer of phosphor positioned between two electrodes, and at least one of the electrodes is light-transmissive. At least one dielectric also is positioned between the electrodes so the EL lamp functions essentially as a capacitor. When a voltage is applied across the electrodes, the phosphor material is activated and emits a light.
EL lamps are typically manufactured as discrete cells on either rigid or flexible substrates. One known method of fabricating an EL lamp includes the steps of applying a coating of light-transmissive conductive material, such as indium tin oxide, to a rear surface of polyester film, applying a phosphor layer to the conductive material, applying at least one dielectric layer to the phosphor layer, applying a rear electrode to the dielectric layer, and applying an insulating layer to the rear electrode. The various layers may, for example, be laminated together utilizing heat and pressure. Alternatively, the various layers may be screen printed to each other. When a voltage is applied across the indium tin oxide and the rear electrode, the phosphor material is activated and emits a light which is visible through the polyester film.
Typically, it is not desirable for the entire EL polyester film to be light emitting. For example, if an EL lamp is configured to display a word, it is desirable for only the portions of the EL polyester film corresponding to letters in the word to be light emitting. Accordingly, the indium tin oxide is applied to the polyester film so that only the desired portions of the film will emit light. For example, the entire polyester film may be coated with indium tin oxide, and portions of the indium tin oxide may then be removed with an acid etch to leave behind discrete areas of illumination. Alternatively, an opaque ink may be printed on a front surface of the polyester film to prevent light from being emitted through the entire front surface of the film.
Fabricated EL lamps often are affixed to products, e.g., panels, and watches, to provide lighting for such products. For example, EL lamps typically are utilized to provide illuminated images on display panels. Particularly, and with respect to a display panel, EL lamps are bonded to the front surface of the display panel so that the light emitted by the phosphor layers of such lamps may be viewed from a position in front of the panel.
Non-Uniform Illumination of Large Panel Areas:
Heretofore, electroluminescent (EL) display panels having lamp areas larger than about 20 square inches exhibit undesirable variations in light output over the surface of the panel. More specifically, the light output of a given EL panel fades noticeably from the center of the panel out to the periphery thereof, when driven by previously existing controllers.
A single, fixed frequency of 400 Hz or greater is typically employed by prior art EL panel controllers (drivers). For example, ENZ-Electronic AG (Gais, Switzerland) manufactures a number of different EL panel drivers intended for panel sizes (areas) ranging from 20 cm2 to 1000 cm2 (approximately 30 sq. in. to 155 sq. in.). Output frequencies for these drivers range from 200 Hz to 2800 Hz, with no apparent correlation between panel area and drive frequency. For example, various models of these drivers intended for 1200 cm2 panels generate single frequencies ranging between 300 Hz and 800 Hz; drivers for 200 cm2 panels generate single fixed frequencies ranging between 400 Hz and 1500 Hz; while a driver for an 850 cm2 panel generates a frequency of 2800 Hz.
Any one of the previously available controllers is limited in its ability to uniformly illuminate a range of electroluminescent panels having areas of differing sizes.
Furthermore, variations in resistance and capacitance of the illumination and dielectric layers are inevitable in the panel printing process. In addition, as a panel ages, the electrical characteristics of the panel change. As a result, the panel light output is not constant over a period of time when driven by previously existing controllers.
Therefore, what is desired is a smart controller to drive an electroluminescent panel to produce an optimized uniform light output for a variety of illumination areas, capacitances, and resistances.
In accordance with one aspect of the present invention, it was observed that the light output of electroluminescent (EL) devices is a function of not only the applied voltage and the illuminated area, but also a function of the frequency applied to the EL device. More specifically, it was noted that the light output of the EL panel begins to fade from the center of the circle out to the perimeter as the frequency is increased. This effect is mainly due to the RC time constant of the circuit, i.e., at higher frequencies, the effective capacitance of the circuit is not able to completely charge during the half cycle of the excitation wave.
The present invention includes a smart controller which compensates for a range of RC time constants and adjusts its output frequency accordingly such that a relatively uniform light output is obtained on all illuminated areas of large EL display panels. The controller incorporates a device which monitors the current as the pulse train is applied to the EL panel. A sensing circuit determines if the current decayed to near zero or about at least 60% of its initial value during the positive portion of the pulse. If the current did decay to near zero at a fixed base frequency (e.g. 400 Hz), no frequency adjustment is made. If it did not, the frequency is decreased until a decay to near zero current zero is sensed. This self-adjustment may be performed automatically and very rapidly through a continuous feedback loop so as not to be noticeable to the eye. Thus, uniform light output is maintained from panel to panel, regardless of RC time constant differences between different EL panels, or variations in panel electrical characteristics over time.
In an alternate embodiment of the present invention, several different frequencies are applied to the EL panel simultaneously. In this case the frequency that effectively controls the light output is determined by the RC time constant of the EL panel circuit. For higher time constants, a lower frequency is operational, for lower time constants a higher frequency is operational.
Intelligent controllers such as those described above are thus useful in maintaining a relatively constant light output as the panel ages and the RC time constant increases.
In one embodiment of the present invention, an electroluminescent panel includes an electroluminescent lamp formed integrally therewith. The electroluminescent lamp is formed on the panel by utilizing the panel as a substrate for the EL lamp. The panel is fabricated by utilizing the steps of screen printing a rear electrode to a front surface of the panel, screen printing at least one dielectric layer over the rear electrode after screen printing the rear electrode to the panel, screen printing a phosphor layer over the dielectric layer to define a desired area of illumination, screen printing a layer of indium. tin oxide ink to the phosphor layer, screen printing an outlining electrode layer to the panel that outlines the rear electrode, screen printing an outlining insulating layer to the outlining electrode layer, screen printing a background layer onto the panel so that the background layer substantially surrounds the desired area of illumination, and applying a protective coat over the indium tin oxide ink and background layer. The rear electrode of each lamp is screen printed directly to the front surface of the panel, and the other layers of the EL lamp are screen printed over the rear electrode.
The above described method provides an illuminated panel that does not require coupling prefabricated EL lamps to the panel. Such method also facilitates applying the various layers of the EL lamps to the EL substrate as a forward image and, alternatively, as a reverse image.